Willamette River - Geology

Geology

The Willamette River basin was created primarily by plate tectonics and volcanism and was altered by erosion and sedimentation, including some related to enormous glacial floods as recent as 13,000 years ago. The initial trough-like configuration was created about 35 million years ago as a forearc basin while the Pacific Plate subducted beneath the North American Plate. Marine deposits on top of older volcanics underlie the valley, which was initially part of the continental shelf, rather than a separate inland sea. About 20 to 16 million years ago, uplift formed the Coast Range and separated the basin from the Pacific Ocean.

Basalts of the Columbia River Basalt Group, from eruptions in eastern Oregon, flowed across large parts of the northern half of the basin about 15 million years ago. They covered the Tualatin Mountains (West Hills), most of the Tualatin Valley, and the slopes of hills further south, with up to 1,000 feet (300 m) of lava. Later depositions covered the basalt with up to 1,000 feet (300 m) of silt in the Portland and Tualatin basins. During the Pleistocene, beginning roughly 2.5 million years ago, volcanic activity in the Cascades combined with a cool, moist climate to produce further heavy sedimentation across the basin; braided rivers created alluvial fans spreading down from the east.

Between about 15,500 and 13,000 years ago, the Missoula Floods—a series of large outpourings originating at Glacial Lake Missoula in Montana—swept down the Columbia River and backfilled the Willamette watershed. Each flood produced "discharges that exceeded the annual discharge of all the present-day rivers of the world combined". Filling the Willamette basin to depths of 400 feet (120 m) in the Portland region, each flood created a temporary lake, Lake Allison, that stretched from Lake Oswego to near Eugene. The ancestral Tualatin Valley, part of the Willamette basin, flooded as well; water depths ranged from 200 feet (61 m) at Lake Oswego to 100 feet (30 m) as far upstream (west) as Forest Grove. Flood deposits of silt and clay, ranging in thickness from 115 feet (35 m) in the north to about 15 feet (4.6 m) in the south, settled from this muddy water to form today's valley floor. The floods carried Montana icebergs well into the basin, where they melted and dropped glacial erratics on the land's surface. These rocks, composed of granite and other materials common to central Montana but not to the Willamette Valley, include more than 40 boulders, each at least 3 feet (0.9 m) in diameter. Before being partly chipped away and removed, the largest of these originally weighed about 160 short tons (150 t).

The northern part of the watershed is underlain by a network of faults capable of producing earthquakes at any time, and many small quakes have been recorded in the basin since the mid-19th century. In 1993, the Scotts Mills earthquake—the largest recent earthquake in the valley, measuring 5.6 on the Richter scale—was centered near Scotts Mills, about 34 miles (55 km) south of Portland. It caused $30 million in damage, including harm to the Oregon State Capitol in Salem. Evidence suggests that massive quakes of 8 or more on the Richter scale have occurred historically in the Cascadia subduction zone off the Oregon coast, most recently in 1700 CE, and that others as strong as 9 on the Richter scale occur every 500 to 800 years. The basin's high population density, its nearness to this subduction zone, and its loose soils, which tend to amplify shaking, make the Willamette Valley especially vulnerable to damage from strong earthquakes.